1. Field of the Invention
The present invention relates to an image reading device applied mainly to an analog copying machine, a digital copying machine and the like.
2. Description of the Prior Art
Conventionally, a reading device of this type includes a contact glass on which an original sheet is set. An exposure lamp irradiates and scans a face of the original sheet and a photosensitive element is provided below the contact glass. A first mirror reflects the light emitted by the exposure lamp and reflected off of the original sheet in a direction parallel with an original scanning direction, a second mirror reflects the light from the first mirror, and a third mirror reflects the light from the second mirror in a direction opposite to that caused by the second mirror. A lens refracts the light from the third mirror and images the light on the photosensitive element. A first moving frame is provided for mounting the lamp and the first mirror, and is movable in the original scanning direction. A second moving frame is provided for mounting the second and third mirrors, and is movable following the first moving frame. A single motor pulls the first and second moving frames through wire, so that the light reflected from the original sheet is directed to the photosensitive sheet to make an image on the same, and the optical length from the contact glass to the lens remains unchanged as the image is read. See, for example, U.S. Pat. No. 4,344,696).
However, in the conventional image reading device, the first and second moving frames are actuated by a single motor through a pulling wire, and hence when the image of the original sheet has to be moved into focus on the photosensitive element, a separate unit by which the first or second moving frame is fixed to the pulling wire must be mechanically displaced to change a distance between the first and second moving frames. Such a required operation is laborious, and additionally, the focusing operation needs the skill and much time.